Positron emission tomography (PET) is a nuclear imaging methodology that detects pairs of gamma rays emitted indirectly by a positron-producing radionuclide. Because the two emitted gamma rays travel in exactly opposite directions, it is possible to locate their site of origin and thereby reconstruct a three-dimensional image of all positron emitters from a computer analysis of the origins of emitted gamma rays.
Vitamin receptors are overexpressed on certain cells, including many cancer cell types, activated macrophages, and activated monocytes. In particular, folate receptors are overexpressed on many cancers. The folate receptor, a 38 KD GPI-anchored protein that binds the vitamin folic acid with high affinity (<1 nM), is overexpressed on many malignant tissues, including ovarian, breast, bronchial, and brain cancers. It is estimated that 95% of all ovarian carcinomas overexpress the folate receptor. In contrast, with the exception of kidney, choroid plexus, and placenta, normal tissues express low or non-detectable levels of the folate receptor. Most cells also use an unrelated reduced folate carrier to acquire the necessary folic acid.
Following receptor binding of vitamins to vitamin receptors, such as folic acid and analogs and derivatives of folic acid to folate receptors, rapid endocytosis delivers the vitamin into the cell, where it is unloaded in an endosomal compartment at lower pH. Importantly, covalent conjugation of small molecules, proteins, and even liposomes to vitamins and other vitamin receptor binding ligands does not block the ability of the ligand to bind to its receptor, and therefore, such conjugates can readily be delivered to and can enter cells by receptor-mediated endocytosis. Accordingly, imaging agents can be targeted to vitamin receptors, including the folate receptor, for delivery into vitamin receptor expressing cells.
The prostate is a male reproductive organ that functions to produce and store seminal fluid, which provides nutrients and fluids for the survival of sperm introduced into the vagina during reproduction. Like other tissues, the prostate gland may develop either malignant (cancerous) or benign (non-cancerous) tumors. Prostate cancer is reportedly one of the most common male cancers in western societies, and is the second leading form of malignancy among American men.
Prostate-specific membrane antigen (PSMA) is a biomarker that is overexpressed on prostate cancer cells. PSMA is over-expressed in malignant prostate tissues when compared to other organs in the human body such as kidney, proximal small intestine, and salivary glands. PSMA is also expressed on the neovasculature within many non-prostate solid tumors, including lung, colon, breast, renal, liver and pancreatic carcinomas, but not on normal vasculature. PSMA is a type II cell surface membrane-bound glycoprotein with ˜110 kD molecular weight, including an intracellular segment (amino acids 1-18), a transmembrane domain (amino acids 19-43), and an extensive extracellular domain (amino acids 44-750). Though the functions of the intracellular segment and the transmembrane domains are currently reported to be insignificant, the extracellular domain is involved in several distinct activities. For example, PSMA plays a role in the central nervous system, where it metabolizes N-acetyl-aspartyl glutamate (NAAG) into glutamic and N-acetyl aspartic acid. PSMA also plays a role in the proximal small intestine where it removes γ-linked glutamate from poly-γ-glutamated folate and α-linked glutamate from peptides and small molecules.
Though the particular function of PSMA on prostate cancer cells remains unresolved, PSMA is known to undergo rapid internalization into the cell, similar to cell surface bound receptors like vitamin receptors. PSMA is internalized through clathrin-coated pits and subsequently can either recycle to the cell surface or enter lysosomes. Accordingly, imaging agents can be targeted to PSMA for delivery into PSMA expressing cells, such as prostate cancer cells.